Certain cathode-ray tubes employed in, for example, high resolution graphics display systems, include a bipotential lens structure for focusing an electron beam as it propagates toward a display screen. The bipotential lens structure typically includes an overlapping pair of electrically isolated, cylindrical electrodes. A potential difference applied between the cylindrical electrodes generates an electric field that directs electrons in the beam toward the central longitudinal axis of the tube, thereby to focus the electron beam as it propagates toward the display screen.
A cathode-ray tube includes an evacuated glass envelope within which the electron beam propagates along the central longitudinal axis from an electron gun toward the display screen. In one type of bipotential lens structure, the pair of cylindrical electrodes are formed by a metallic cylinder electrode positioned within the neck portion of the glass envelope and an electrically resistive coating on an interior surface of the neck portion. The resistive coating partly overlaps the metallic electrode and is itself overlapped by a magnetic deflection yoke positioned outside the evacuated envelope. The deflection yoke scans the electron beam across the display screen in a raster pattern.
A bipotential lens that employs a resistive coating as one of the cylindrical electrodes is desirable because it is relatively inexpensive to manufacture. Such a lens structure suffers, however, from the disadvantage of causing the evacuated envelope to rupture when an electric arc of sufficiently high current develops between the metallic electrode and the resistive coating.
In particular, the lens structure generates large electric field gradients near the end of the resistive coating where it partly overlaps the metallic electrode. Whenever an arc occurs between the metallic electrode and the resistive coating, a large electric current at a relatively high voltage is delivered through the coating. The impedance of the resistive coating, together with the electric field gradients near its end, causes the current in the arc to be localized on the surface of the glass envelope near the end of the coating.
The relatively large, localized current in an arc raises the temperature of the glass envelope, and the increased temperature of the glass envelope increases its conductivity. As a result, the temperature of and the current in the glass envelope near the end of the resistive coating increase. Such temperature and current increases can occur until a second arc is generated between the interior surface of the glass envelope and its exterior surface. The second arc, which is called "punch-through", ruptures the glass envelope and thereby destroys the cathode-ray tube.
Uncontrolled arcs between the cylinder electrode and the resistive coating may occur during normal operation of the cathode-ray tube or during the conditioning of the cylinder electrode to eliminate field emission locations on its surface. Such conditioning is a processing step in the manufacture of cathode-ray tubes and is called "spot knocking." During the spot knocking process, the field emission locations (e.g., contamination on the surface of the metallic electrode) are eliminated by generating current-controlled arcs between the metallic electrode and the resistive coating. Typically, the arc current is selected so that it is sufficient to "burn-off" the field emission locations but is insufficient to cause punch-through.
FIG. 1 is a schematic longitudinal section view of a prior art bipotential electron lens structure 10 positioned in a glass envelope 12 of a cathode-ray tube 14. Bipotential lens 10 includes an inner cylindrical electrode 16 and a partly overlapping outer cylindrical electrode 18 that are axially aligned with a central longitudinal axis 20. Outer cylindrical electrode 18 is supported within a neck portion 22 of glass envelope 12 by a pair of snubbers 24a and 24b, which provide an electrical connection between outer cylindrical electrode 18 and an electrically resistive coating 26 on the interior surface 28 of envelope 12. Resistive coating 26 is overlapped by a magnetic deflection yoke 30 positioned outside glass envelope 12.
Outer cylindrical electrode 18 includes a particle trap 32 to which snubbers 24a and 24b are attached, as described, for example, in U.S. Pat. No. 4,665,340 of Odenthal et al. for "Cathode-Ray-Tube Electrode Structure Having a Particle Trap", issued May 12, 1987. Trap 32 includes a metal disk 34 that extends across neck portion 22 of envelope 12. An axially aligned central aperture 36 in metal disk 34 includes a cylindrical axial flange 38 that extends toward the display screen (not shown) of tube 14.
Particle trap 32 and snubber 24a extend completely across neck portion 22 of envelope 12 to provide a "cup-like" configuration that collects particles propagating from a funnel portion 40 of tube 14. Such particles may include, for example, contamination that is dislodged from funnel portion 40 and that could establish field emission points on inner electrode 16, or secondary electrons that are emitted from funnel portion 40 and that could provide the current to support an uncontrolled arc between electrodes 16 and 18.
Bipotential lens structure 10 reduces the incidence of "punch-through" because an arc between electrodes 16 and 18 does not directly contact interior surface 28 of envelope 12. It will be appreciated, however, that the manufacture of particle trap 32 is relatively expensive compared to a bipotential lens employing a resistive coating alone. In addition, the combined width 42 of outer cylinder electrode 18 and snubbers 24a and 24b allows eddy currents to be generated therein by deflection yoke 30. Such eddy currents draw energy from the deflection fields generated by yoke 30 and thereby reduce the power with which it interacts with the electron beam.